1. Field of the Invention
The present invention relates to a projection exposure method and the apparatus thereof, and more particularly, to a projection exposure method and projection exposure apparatus used in manufacturing processes of fine circuit patterns, semiconductor integrated circuits, liquid crystal displays, etc.
2. Discussion of the Related Art
In the photolithographic processes for manufacturing semiconductor devices, liquid -crystal display devices, etc., a projection exposure apparatus equipped with a projection optical system has been used to project the pattern image on a photomask (or reticle, generally referred to as "reticle") onto each shot region of a substrate, such as a wafer or glass plate, (generally referred to as "wafer") coated with a photosensitive material. In recent years, a step-and-repeat type exposure apparatus, in particular, a reduction projection type exposure apparatus (stepper) has become more and more popular. In the step-and-repeat type exposure apparatus, the wafer is held on a wafer stage which is movable in a two-dimensional plane. The wafer stage moves stepwise to successively expose the shot areas of the wafer with the pattern image of the reticle.
A semiconductor device or the like is, in general, made of a plurality of patterned layers on the wafer, each of which is formed using photolithography processes involving exposure operations. In such projection exposure operations, it is important to accurately align the circuit patterns already formed on the wafer with a pattern image of the reticle to be transcribed. Thus, alignment of the wafer and the reticle needs to be performed with high precision.
One method for performing such alignment is as follows. A wafer alignment mark on the wafer is illuminated with a light beam having the same wavelength as exposing light (exposure wavelength) to project the image thereof onto the patterned surface of the reticle adjacent a reticle alignment mark formed on the reticle. Then, a positional relationship between the image of the wafer alignment mark and the reticle alignment mark is directly measured to derive a relative positional relationship between the wafer and the reticle and to thereby perform alignment. This method is referred to as the "reticle reference method." However, this reticle reference method has a disadvantage in that the wafer alignment mark and its neighborhood on the wafer are exposed with the light beam of exposure wavelength. Because of this, the reticle reference method is not presently used.
In a more popular alignment method, the position of the alignment mark on the wafer is detected using a light beam having a different wavelength from that for exposure. Examples of this type of alignment method are a through-the-lens (TTL) method, in which a projection optical system is used as a portion of an optical system detecting the wafer alignment mark, and an off-axis method, in which a separate optical system is used to detect the position of the wafer mark. In these systems, the reticle and wafer are not aligned directly, but aligned indirectly using a reference mark (fiducial mark) provided in the projection exposure apparatus (normally on the wafer stage holding the wafer).
The off-axis method is explained in detail as an example. First, prior to exposure, the above-mentioned fiducial mark, formed on the wafer stage, is aligned with the image of the reticle alignment mark and then the position of the wafer stage is measured. Next, the fiducial mark is moved to a position located below the optical system of a wafer mark detection system and is aligned with a detection reference of the optical system for the wafer mark detection system. Then, the position of the wafer stage is measured. The difference between these two wafer stage positions is referred to as "baseline amount," and the above-mentioned sequence is referred to as "baseline measurement."
Before actually exposing the wafer, the wafer stage is positioned such that a position detection mark formed on the wafer is aligned with the detection reference of the wafer mark detection optical system. Subsequently, the wafer stage is moved to a position displaced by the baseline amount so that the existing circuit pattern on the wafer can be aligned with the image of the reticle pattern.
When a plurality of wafers are sequentially exposed in the conventional method above, the above-mentioned baseline measurement is performed only once at the beginning of the exposure sequence, and the alignment and exposure of the wafers is performed using the baseline amount thus measured. Therefore, a positional relationship between the reticle projection image and the optical system of the wafer mark detection system is not checked during the exposure sequence for the plurality of wafers of the same kind (during repetition of the same process). In other words, superposition exposure is performed under the assumption that the positional relationship will not change.
However, since the minimum line-width of circuit patterns has become extremely small due to advancements in semiconductor technology, small fluctuations in the above-mentioned positional relationship, which were not a problem in the past, have come to impose significant effects on alignment precision and the performance of the semiconductor integrated circuit manufactured thereby. Moreover, since measurement errors in the baseline amount itself directly contribute to the alignment error, higher accuracy has become a necessity in determining the baseline amount.
For this reason, a method for accurately measuring the baseline amount, which does not have variations over time, is required. Theoretically, when a plurality of wafers are to be exposed, it is possible to measure the baseline amount several times and take an average to determine a precise baseline amount every time a specified number of wafers are exposed.
However, since a processing time of approximately 10 to 20 seconds is required for each baseline measurement, if a plurality of baseline measurements are performed for each exposure of a specified number of wafers as described above, this baseline measurement method leads to a considerable reduction of throughput in the projection exposure apparatus. Therefore, in reality, it is difficult to apply this method.